I wouldn't stagger them like that.

How about this; since you are going to drill and tap them for screw-in bleeds anyway, why not drill the IFRs a bit bigger (say .001" or .002") and use the same wires? Or wire down your IABs. Either way should fatten up your idle a bit. And will give you a chance to see how it acts with each combo and if it likes one better than another. And I would concentrate on the center carb unless the outboard's idle circuits are way screwed up.

And have you checked your slot feed? By 1,500 your throttle plate position should be pulling on the slot a good bit.

Don't get too hung up on idle A/F ratio. Just give it what it likes whether that is in the 13s or the 14s. Get the mix screws in the 1 to 1 1/2 range with a solid idle and call it good. Then go to the slot for off idle and the air bleeds and emulsion holes for main circuit actuation timing.

On a side note, my old outboard carbs had idle mixture screws that were plugged over with lead. Any chance yours do too?